Vitamin D is required for normal skeletal development and maintaining the integrity of bone tissue. The physiological effects of vitamin D are mediated through the vitamin D receptor (VDR), a ligand-activated factor that influences transcription of vitamin D-responsive genes. Central to the transcriptional mechanisms is heterodimerization of VDR with retinoid X receptors (RXRs), a class of the nuclear receptors that mediate vitamin A action. The VDR/RXR heterodimer binds to specific DNA sequences (VDREs) to ultimately influence the rate at which RNA polymerase II transcribes vitamin D responsive genes. The precise mechanism by which VDR/RXR interaction with VDREs alters gene transcription is presently unknown. Therefore, the overall goal of this proposal is to understand the communication process that links the VDR/RXR/VDRE complex with RNA polymerase II. We demonstrated that VDR forms a specific, direct, protein:protein contact with transcription factor IIB (TFIIB). Based on these data, we hypothesize that the VDR/RXR heterodimer communicates with the preinitiation complex through extensive protein:protein contacts and the interaction of VDR with TFIIB is central to the vitamin D-mediated transcriptional process. To test this hypothesis, we propose 4 specific aims that examine: 1) the physiologic and functional relevance of VDR-TFIIB interaction in osteoblast cell lines; 2) the biochemical binding parameters of the VDR/TFIIB complex and the role of RXR, VDRE, and ligands in VDR-TFIIB complex formation; 3) the molecular contacts mediating VDR/TFIIB interactions; and 4) the essentiality of VDR-TFIIB interaction in vitamin D-mediated transcription. These studies are designed to improve our understanding of the fundamental mechanism of transcriptional regulation by vitamin D. In the future, this information will impact on the development of rationale strategies to address a variety of bone and mineral disorders such as osteoporosis.